1. Field of the Invention
The present invention relates to a method of stopping a servo motor, which is driven by a three-phase AC current generated by an inverter circuit, without intervening a control portion for controlling the inverter circuit or directly stopping the servo motor passing through the control portion, and also relates to a stopping apparatus used at the time of implementing the method. In particular, the present invention relates to the method of stopping the servo motor and the servo motor stopping apparatus that are suitably applied to the servo motor required to be safety which is used for a tool machine, a production machine or an industrial machine etc., although not limited thereto.
2. Description of the Related Art
The production machine or the industrial machine is provided with a member which operates with a quite large force and at a high speed to process and transfer material, that is, an operation member. Since a serious accident arises if a worker directly touches such the operation member, a movable protection guard is provided so as to realize the safety measure for preventing an accident.
However, the safety can not be secured sufficiently by merely providing the movable protection guard. To be more in detail, at the time of performing the maintenance of the industrial machine, it is required to perform the maintenance procedure while opening the movable protection guard. During such the maintenance procedure, a required constituent member is applied with a brake so as not to drive a movable portion. Further, such a safety measure is employed that in the case where a driving source is a hydraulic type, a release valve is opened to interrupt the supply of operation oil to a hydraulic cylinder. The similar safety measure is employed when the driving source is a servo motor.
A servo motor employed in the production machine or the industrial machine, that is, a servo motor driven by a three-phase AC current has been provided with three stator windings of a U−phase, a V−phase and a W−phase provided within the motor, three terminals for applying voltages thereto and an encoder for detecting a rotational position, etc. When predetermined voltages are applied to the stator windings of the U−phase, V−phase and W−phase, magnetic field is generated and a torque is applied to the rotary of the servo motor configured by a permanent magnet. Thus, when the polarities and durations of the voltages applied to the U−, V− and W−phases are controlled in accordance with the rotational position detected by the encoder, the motor can be driven and rotated in an arbitrary direction with an arbitrary torque. Although such the voltages are generated by an inverter circuit, the inverter circuit is controlled in accordance with a signal from a control portion. JP-A-8-191591 discloses a method of stopping such the servo motor.
According to the servo motor stopping method described in JP-A-8-191591, in the case of stopping the servo motor being driven and rotated, first, voltages are applied to the U−, V− and W−phases so that a reverse rotation torque is generated in order to rotate the motor in a direction in opposite to the current rotational direction. Then, when the rotation speed reduces to or less than a predetermined speed, DC voltages of predetermined polarities are applied to the U−, V− and W−phases to excite by DC to thereby stop the servo motor.
However, the related art servo motor stopping method has some disadvantages. For example, according to the servo motor stopping method described in JP-A-8-191591, since the reverse rotation torque is generated at first, the servo motor can be decelerated abruptly. Further, when the rotation speed reduces to or less than the predetermined speed, the DC energization is performed to completely stop the motor. Thus, the method has an effect that the servo motor can be stopped in a short time. However, there arises a problem when as the safety method for the production machine etc., the servo motor stopping method is applied to the case where the servo motor is kept in the stopped state. For example, when the DC voltage having been applied to the U−, V− and W−phases is stopped after the servo motor stops, no torque is applied to the servo motor. Thus, since no brake is applied to the servo motor, it becomes impossible to prevent the mechanical member from falling due to the weight of a mechanical member coupled to the servo motor. On the other hand, when the DC voltage is kept to be applied to the U−, V− and W−phases after stopping the servo motor, although the servo motor can be kept in the stopped state, energy is consumed wastefully since an amount of the power consumption is large. Further, since the current flows into the stator windings for a long time, a large amount of heat may be generated, which may cause the breakage of a wire etc.
Further, the servo motor stopping method described in JP-A-8-191591 has a problem that even after stopping the servo motor, the servo motor is continued to be supplied with the power and so continuously controlled by the signal from the control portion. In a state that the power supply to the servo motor is not interrupted, if the control portion generates an abnormal signal, the servo motor is driven.
The control portion is configured by a hardware including a CPU and a memory etc. and a program formed by combining various kinds of instructions, that is, software. Although the reliability of the hardware has been improved recently, there is no guarantee that the hardware does not become faulty. The hardware may catch noise and operate erroneously, that is, runaway. Also as to the software, even if the software is tested sufficiently at the time of the development and the quality thereof is secured, since advanced and complicated programs have been developed in many cases due to the recent tendency of the high-performance of the production machine, it is impossible to completely eliminate the possibility that the program may contain software bug potentially. The control portion may generates an abnormal signal due to such the abnormality of the hardware or software. Thus, the stoppage of the servo motor is not guaranteed and so the safety of workers is not guaranteed.